The invention relates to a tensioner for tensioning a belt of a belt drive system, but more particularly, the invention relates to a damping mechanism for a spring type tensioner where the spring biases the position of a pivot-arm to which a pulley is attached. While the tensioner of the invention may be used in various applications for tensioning belts, it is particularly useful in controlling belt tension of a V-ribbed belt as associated with a front end accessory drive or a synchronous belt as associated with a camshaft belt drive system for automotive applications.
Automatic tensioners of the mechanical type have designs for accommodating a particular type of spring to bias a position of the tensioner's pivot-arm. Various types of springs are known to be used to accommodate a particular design and include belville springs, volute springs, compression springs, tension springs, or torsional springs. Each type of spring offers some advantages while also introducing some design limitations for a tensioner. Perhaps, the spring most used today in automotive tensioners is a coiled torsional spring. An advantage of using a torsional spring is that four forces are needed to define two couples for winding the spring and wherein at least one of the forces can be advantageously used or "tapped" to operate a damping mechanism that inhibits movement of the pivot-arm.
While springs with torsional type tensioners have some design advantages such as the option to use one of the spring winding forces to generate damping, they also have some limitations. For example, the amount of available damping is limited to a function of a radius at which friction surface sliding takes place to effect damping. Also, torsional spring type tensioners must be mounted with its pivot-arm angled in a geometric manner relative to an engaged belt so that a trigonometric shortening and lengthening of the pivot-arm compensates for variations introduced by the coiled torsional spring. An example of a tensioner with a torsional spring and a damping mechanism operative with friction surface sliding at a radius is shown in U.S. Pat. No. 5,632,697.
Some of the disadvantages of using a coiled torsional spring for a tensioner may be overcome with a tensioner designed to use a compression spring. A compression spring may be operatively connected between a base member and a pivot-arm in such a manner that it provides substantially a constant torque output as measured at a pivot-arm. Also, the radius or moment about which friction surface sliding takes place may be increased to effect a larger percentage of damping. Examples of tensioners with compression springs are disclosed in U.S. Pat. Nos. 5,098,347; 5,045,031; and 5,591,094. While such tensioners solve some of the problems associated with torsional springs such as by providing a constant torque output and a larger radius at which damping takes place with friction surface sliding, they also introduce their own disadvantages. One of the disadvantages is that only two forces are available to "tap" with a compression spring as opposed to four forces available for "tapping" with a torsional spring. Consequently, and with only two available forces, there is no force of a compression spring that is easily "tapped" to provide a force for a damping mechanism. More particularly, there is a problem of defining a force for damping that is additive to a hub load to effect a total force available for a damping mechanism. In other words, the prior art tensioners are unable to define a spring force for a damping mechanism in conjunction with a single compression spring that biases a position of a pivot-arm. The prior art solved the spring force problem for damping with the addition of a second spring such as a U-shaped leaf spring or a second compression spring where the springs provide a constant force which results in constant (as opposed to variable) damping of a pivot-arm.
This invention is directed to those tensioner designs that incorporate a compression spring to bias the position of a pivot-arm and to solving the prior art problem of the necessity of using a second spring to provide a force to a damping mechanism to effect damping by friction surface sliding.